1. Field of the Disclosure
The present invention relates generally to control circuits and, more specifically, the present invention relates to control circuits used in power converters that implement multiple operating modes.
2. Background
Power converter control circuits may be used for a multitude of purposes and applications. Due to increasing global requirements for energy efficient power converter solutions, there is increasing demand for control circuit functionality to meet these requirements. In addition, most power converter applications have cost goals. In the case of a control circuit forming part of an integrated circuit, the package in which the integrated circuit is housed can significantly contribute to its cost. The number of pins or terminals that it uses in turn influences the cost of the integrated circuit package. It is therefore desirable for a control circuit to implement control modes that allow energy efficiency goals to be met in a package with low pin count to meet the cost goals of the power converter application.
Energy efficient power converters typically employ a switching power converter configuration in which a switch is coupled to the control circuit and to an energy transfer element. The energy transfer element could be a transformer in power converters such as flyback or forward converters or a simple inductor in other power converter configurations such as buck, Cuk or SEPIC converters. The control circuit controls switching of the switch to regulate energy transferred through the energy transfer element and therefore power delivered to an output of the power converter.
Many power converter control solutions implement two or more control modes. Typically one control mode of operation is implemented for normal load conditions and another mode is implemented under very light load, standby or no-load conditions. A typical control mode of operation for very light load, standby or no-load conditions is referred to as cycle skipping or burst mode control. These modes of control typically maintain the regulation of an output parameter of the power converter by alternating between periods where power is delivered to the output of the power converter and periods where the power delivered to the output of the power converter to substantially zero. In this type of control mode, the period of time for which substantially zero power is delivered to an output of the power converter is regulated in order to maintain regulation of an output parameter, typically an output voltage appearing across output terminals of the power converter. These modes of control improve the efficiency and therefore reduce the energy consumption of the power converter since the periods for which substantially zero power is delivered to the power converter output are typically achieved by suspending switching of the switch coupled to the energy transfer element. Switching losses associated with the switching of the switch are therefore eliminated for the period for which switching is suspended.
This burst mode or cycling skipping mode of operation is well suited to very light load, standby or no-load conditions. However, emerging energy efficiency standards are requiring that stringent power converter efficiency goals are achieved over a wide range of load conditions. At higher load conditions the cycle skipping or burst mode control schemes described above are less effective since they tend to lead to power converter instability, increased power converter output voltage ripple and potentially audible noise.
At higher load conditions therefore pulse width modulation (PWM) control or duty cycle control schemes are better suited. For the purpose of this disclosure, these PWM or duty cycle control schemes are defined as those schemes that do not require cycle skipping to regulate power delivered to an output of the power converter. Such control schemes avoid the need to skip switching cycles by instead controlling various other parameters to regulate the power delivered to the output of the power supply. For example, voltage mode control employs a fixed switching frequency and varies an on-time of the switch during each switching period. The switching frequency being the reciprocal of one switching cycle period of the switch. The ratio of the switch on-time to the overall period of the switching cycle is referred to as the duty cycle.
Another common control scheme is current mode control that employs a fixed switching frequency and varies a current limit threshold of a current flowing in the switch while the switch is on. By regulating the current limit threshold, the energy per cycle is regulated and in turn the power delivered to the output of the power converter is regulated.
Yet another example of a PWM or duty cycle control scheme is fixed on time variable off time. Although the variable off time of this control scheme gives rise to changes in switching cycle time, it still fits the definition of a PWM or duty cycle control mode of operation given above in that the switching cycle time period is gradually changed from one switching cycle to the next to regulate the power delivered and no cycles are skipped.
Still other control schemes such as fixed off time variable on time, resonant mode and quasi-resonant mode all regulate power delivered to the output of the power converter without skipping cycles, but instead by varying the switch on-time pulse width as a proportion of a switching cycle period. Again, the switch on-time as a proportion of a switching cycle period is referred to as the duty cycle.
In each case, although the control schemes may result in changes in switching period over a number of switching cycles, they fit the definition of a duty cycle control mode of operation for this disclosure as they do not skip cycles and the switching cycle time period is changed gradually over a number of cycles to regulate the power delivered. For the purposes of this disclosure therefore, these various control schemes that regulate the power delivered through varying the switch on time as a proportion of the switching cycle time period without skipping cycles will be referred to as duty cycle control modes of operation. It is understood that an alternative description for these operating modes could be PWM modes of operation.
One key challenge in implementing multiple duty cycle control modes of operation is ensuring a smooth transition between operating modes. The transition between modes normally introduces some change or discontinuity in the control loop gain of the power converter since each duty cycle control operating mode has distinct characteristics in term of control loop gain depending on the operating conditions of the power converter. Known solutions employ hysteresis when transitioning between duty cycle control operating modes to ensure that any change in the control loop gain during the transition from one control mode to another control mode, does not result in control loop instability potentially causing oscillations between modes, which could give rise to power converter output voltage ripple, audible noise and even damage to certain components within the power converter.
Another key challenge in implementing multiple duty cycle control modes of operation is to maintain a low cost solution. The control circuit should require the minimum number of terminals to implement the multiple operating modes. Known solutions implementing multiple duty cycle control operating modes employ additional terminals to sense the load condition at which to transition from one duty cycle control mode to another. Such terminals are coupled to an additional winding on the energy transfer element of the power converter to detect the period of energy delivery during each switching cycle which changes with output load. Such implementations perform a bottom detect (BD) scheme to sense when a voltage on the additional energy transfer element winding rings to a low voltage level, which is an indication that energy delivery to the power converter output is complete for that switching cycle.
It is therefore desirable to have a control circuit that maintains high power converter efficiency across a very wide load range by implementing multiple duty cycle control modes, while employing low cost packaging to maintain a low cost power converter design.